Seeley attended Cambridge University, but quit before earning a degree. He began working as an assistant to Adam Sedgwick at the Woodwardian Museum in 1859 and was later offered positions with both the British Museum and the Geological Survey of Britain. Rather than accept either, he worked on his own, and only accepted a position with King's College much later in life.
In 1887, Seeley divided dinosaurs the Ornithischia and Saurischia based on the pelvic structure of dinosaurs known at the time. At the time Seeley also argued for argued for the separate origins of the groups even though O.C. Marsh had identified many characteristics common to all dinosaurs.
Seeley was also an authority of pterosaurs, and in 1901 published a popular book on the subject, Dragons of the Air.
Info from HERE. Image from HERE
